Application of specific electrical energy to the spinal cord for the purpose of managing pain has been actively practiced since the 1960s. While a precise understanding of the interaction between the applied electrical energy and the nervous tissue is not fully appreciated, it is known that application of an electrical field to spinal nervous tissue can effectively mask certain types of pain transmitted from regions of the body associated with the stimulated tissue. More specifically, applying particularized electrical pulses to the spinal cord associated with regions of the body afflicted with chronic pain can induce paresthesia, or a subjective sensation of numbness or tingling, in the afflicted bodily regions. This paresthesia can effectively inhibit the transmission of non-acute pain sensations to the brain.
Electrical energy, similar to that used to inhibit pain perception, may also be used to manage the symptoms of various motor disorders, for example, tremor, dystonia, spasticity, and the like. Motor spinal nervous tissue, or nervous tissue from ventral nerve roots, transmits muscle/motor control signals. Sensory spinal nervous tissue, or nervous tissue from dorsal nerve roots, transmit pain signals. Corresponding dorsal and ventral nerve roots depart the spinal cord "separately"; however, immediately thereafter, the nervous tissue of the dorsal and ventral nerve roots are mixed, or intertwined. Accordingly, electrical stimulation intended to manage/control one condition (for example, pain) often results in the inadvertent interference with nerve transmission pathways in adjacent nervous tissue (for example, motor nerves).
Electrical energy is commonly delivered through electrodes positioned external to the dura layer surrounding a spinal cord. The electrodes are carried by two primary vehicles: the percutaneous lead and the laminotomy lead. As the present invention primarily concerns the placement of percutaneous leads (i.e., any lead which may be positioned within an epidural space using an external insertion needle), a further discussion or description of laminotomy leads (i.e., leads which require surgical removal of vertebral material to allow access to an epidural space) will not be provided here.
Percutaneous leads commonly have two or more electrodes and are positioned within an epidural space through the use of an insertion, or Touhy-like, needle. An example of an eight-electrode percutaneous lead is an OCTRODE.RTM. lead manufactured by Advanced Neuromodulation Systems, Inc. of Allen, Tex.
Operationally, an insertion needle is passed through the skin, between the desired vertebrae, and into an epidural space which is defined by a dural layer in combination with the surrounding vertebrae. The stimulation lead is then fed through the bore of the insertion needle and into the epidural space. Conventionally, the needle is inserted at an inferior vertebral position, for example, between vertebrae L1 and L2 (L1/L2)(see FIGS. 1a and 1b), and the stimulation lead is advanced in a superior direction until the electrodes of the stimulation lead are positioned at a desired location within the epidural space, for example, at T10. In a lateral position, percutaneous leads are typically positioned about a physiological midline.
As an example of application, the above methodology is commonly used for the management of sympathetically maintained pain (SMP). It is generally believed that due to the sympathetic nature of SMP, stimulation leads positioned about a physiological midline provide sufficient electrical energy to interrupt the transmission of SMP signals. This may be because sympathetically maintained-type pain requires less nervous fiber selection and/or is less susceptible to interference by interpositioned cerebrospinal fluid.
The above-described conventional technique is used less often for the management of sympathetically independent pain (SIP). SIP is somatic in nature or a mixture of somatic and sympathetic. While it is recognized that SIP could potentially be managed by conventional implantation methods, there currently exists an inability to consistently achieve either a required level of nervous fiber selection or adequate stimulation through interpositioned cerebrospinal fluid at the stimulation site. Consequently, to manage SIP, electrical energy is commonly delivered to the spinal nerve roots corresponding to the pain-afflicted dermatomes. Using conventional implantation methods, electrical energy can only be applied to nerve roots by placing a percutaneous lead in lateral extremes of the epidural space, or in the epidural "gutters" (see FIGS. 2a and 2b). Of note, however, a percutaneous lead inserted at an inferior location and advanced in a superior direction cannot reach the epidural gutters above C2/C3 of a vertebral column.
As seen in FIG. 2b,positioning a stimulation lead in this manner results in the electrode portion of the stimulation lead spanning a plurality of nerve roots. To stimulate the correct nerve root(s), it is critical that the applied electrical energy be properly defined. An improperly defined electric field may not only be ineffective in controlling/managing the desired condition(s) but may also inadvertently interfere with the proper neural pathways of adjacent spinal nervous tissue.
An applied electrical field is defined by the polarity of each electrode of the stimulation lead. Conventionally, each electrode is set as an anode (+), cathode (-), or neutral (off). As may be understood, for a four electrode percutaneous lead there exists approximately 50 electrode combinations. For an eight electrode percutaneous lead, the number of possible electrode combinations grows to approximately 6050.
Utilizing conventional implantation techniques, a user must rely solely upon effectively programming the electrodes of a multiple electrode percutaneous catheter to define an electric field for "selection" of spinal nervous tissue to either inhibit the transmission of pain signals to the brain or control the symptoms of a motor/muscular disorder. Unfortunately, the time required to identify/define an optimum electric field may be prohibitive.
As an alternative to spinal cord stimulation, electrical energy may be delivered to selected peripheral nerves using a peripheral nerve stimulation system. Peripheral nerve stimulation involves administration of electrical energy to a localized group of peripheral nerves through placement of one or more leads at the peripheral nerve site. Unfortunately, if a patient's pain is widespread, a patient may require a plurality of stimulation leads to be implanted. The surgical procedure necessary for stimulation lead implantation is significant and can be quite painful. Additionally, because peripheral stimulation leads are implanted in "active" areas of the body (e.g., arms and legs), the leads typically lack long-term placement stability. Lead movement, or lead migration, can affect the quality of pain relief. Further, significant lead movement that undermines the intended stimulation effect may require additional corrective surgeries to reposition the stimulation leads.
Accordingly, a need exists for a technique that enables the effective placement of multiple electrode stimulation leads which allows "selection" of desired spinal nervous tissue to manage chronic pain and/or symptoms of motor dysfunction.